I don't know but "joints not listed" sounds odd to me. I can't imagine why something that works when applied to some joints wouldn't work for some on all joints. I can see why they would say it doesn't work for everyone or all the time. I guess maybe some joints could be less prone to the certain problems that they know it works for and that's what they mean. But seems strange the maker would say it that way. If it doesn't say don't use on this or that joint I don't know why it could hurt to give it a try anywhere. That said the spine is unique and along with one or two others is a very different make up than most joints in the body. That wasn't very helpful was it. Sorry
I went back and checked the website I was reading and what it said Was "Voltaren gel is used to treat pain in the hands, wrists, elbows, knees, ankles, or feet caused by osteoarthritis. This medication may not be effective in treating arthritis pain elsewhere in the body." Sorry I read it wrong the first time. I was checking into it on Drugs.com.
I use it all the time on my C6-T2 & L4-S1 & SI joints. I have osteoarthritis in these areas. It works fine for me most of the time. But if I'm having a fibromyalgia flare, it feels like it doesn't touch the Lumbosacral areas. I use it a lot on my calf muscles that tend to cramp up at bedtime. If I'm getting a headache room the C6-T2 area it barely helps at all. 🙏🌼